Control of the actin cytoskeleton is central to many processes in eukaryotic cells such as polarity establishment, motility, and cell division. In the Drosophila oocyte, two actin regulators, Spire (Spir) and Cappuccino (Capu), cooperate during oogenesis to determine the timing of cytoplasmic streaming, a dynamic process, critical to proper development. The molecular mechanism is not well understood. Spir and Capu interact directly, and both proteins nucleate new actin filaments. The goal of this research is to understand the activity, structure, and regulation of the protein complex formed by Spir and Capu. Two approaches will be used to examine the activity and dynamics of the complex: single filament imaging and bulk actin polymerization assays. The questions these experiments answer include: how do Spir and Capu regulate each other's activity? Does the Capu:Spir complex remain bound after nucleation? What is the activity of the complex in the presence of two additional interaction partners, profilin and microtubules? X-ray crystallography will be used to obtain atomic-resolution structural data for this novel protein-protein interaction. This structure will have a significant impact on our understanding of how these proteins work together. Finally, investigations of the regulation of Spir and Capu will focus on whether either protein is auto-inhibited and, if so, how the interaction between the two proteins affects auto-inhibition. The results of this research will lead to a model for how Spir and Capu work together in the oocyte to control the timing of cytoplasmic streaming. Based on their overlapping expression patterns in the mouse embryo, the importance of a Capu homolog in mammalian fertility, and a physical interaction between the human homologs of Spir and Capu, I am confident that any model developed for Drosophila Spir and Capu should have general relevance for human health. I am investigating the mechanism of two proteins that build large structural networks in the developing fruit fly egg. Human cells have closely related proteins that are important for fertility in mammals.